NSW and Victoria lagged behind the smaller states of Queensland and South Australia, where about one in five people were found to have prepared an advance care document. Both states have long-standing, well-publicised statutory forms allowing a person to accept or refuse treatment, the study found.

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Study author Ben White said advance directives, which are documents outlining a person's wishes if they lose capacity to consent to or refuse treatment, took the pressure off medical teams and families.

"The doctors have their own point of view, but they want to take into account the patient's point of view as well, and that largely means it will fall on the family," he said. "In Australia the issue of over-treatment is a live issue, and advance care planning is a way to try to tackle that."

Professor White, the director of the Australian Centre for Health Law Research at the Queensland University of Technology, said advance care planning needed to be embedded in aged care and hospital settings, and there could be the potential to train lawyers and financial planners to raise the issue and encourage consultation with doctors.

His study, conducted in conjunction with researchers at the University of Queensland, found people who had appointed a financial enduring power of attorney were nine times more likely to have an advance directive. Those with a will were 2.5 times more likely.

Single people or those not in a legally recognised relationship were also more likely to have outlined their wishes.

Amy Waller, a research fellow at the University of Newcastle who is examining advance directives for cancer patients, said even when people were very unwell they were generally still not outlining their medical wishes.

"It's not that people don't like having those conversations, more that they don't know how, and then it becomes too late because the person is quite sick," she said. "Not everybody is going to want to have these conversations – some are quite happy to leave it to their doctor – but it is really important to give them the option."

In NSW advance directives are recognised under common law "and the absence of legislative backing for an endorsed, standardised form may affect community awareness and acceptance of advanced directives", according to the research, published online in the Internal Medicine Journal.

In Victoria a statutory advance directive form is available but it only refers to the refusal of treatment in the case of a particular condition.